This invention relates generally to magnetic resonance imaging systems and more particularly to fabricating gradient coil arrays combined with surface RF coils, and torque-balanced gradient coils.
At least some known MRI scanners use surface RF coils to enhance imaging performance of the scanners. When using MRI for functional imaging or neurological imaging it is desirable to have strong, rapidly switched gradient fields in order to achieve high resolution both spatially and temporally. Local RF coils, or surface coils, are alternatives to RF volume coils for imaging the spine and brain stem. However, surface RF coils may exhibit an inhomogeneous RF field that affects image uniformity, and makes them generally undesirable for use as a transmitter, but may also provide an improved signal-to-noise ratio. For at least some known applications, local insertable MRI gradient coils offer a compromise among the competing performance requirements of linearity, amplitude and slew rate, limited dB/dt, and patient accessibility. Gradient coil designs that are single-sided and flat or slightly curved can be particularly accessible. When this type of gradient coil is operated within a known cylindrical magnet MRI system, the current that produces the magnetic field gradient along the magnet axis, for example, the z-gradient coil, produces a net torque. This is due to a required asymmetry of a transverse component, designated x, of the current. The asymmetry is required in order to produce a field that has the appropriate variation with displacement along z. Assuming a uniform main field, the asymmetry results in unbalanced Lorentz forces directed along the other transverse direction, y, resulting in a net torque about the axis of imbalance, x-axis. Therefore, there is a net torque on the whole gradient coil assembly.
Furthermore, the net torque increases in magnitude with increasing current density in the coil. So, as the gradient coil is operated at higher currents so as to take fuller advantage of the imaging capabilities of the coil, the imaging system is increasingly vulnerable to mechanical motion arising from this torque, which may compromise image quality and introduce a concern for patient comfort when proximate to the coils.